The naming convention for each version after version 5.1 changed which can cause confusion as prior versions were named by the version number ES2, ES3, etc.
All versions post ES5.1 follow a yearly naming convention starting with ES2015 which is now incorrectly referred to as ES6. When searching for information on the newer releases, be sure to use the correct naming convention.
ES2015 brought major changes to the standard and added many features, this has been iterated on every year since.
Most browsers support ES5. Older versions of Internet Explorer do not support full ES5 with IE10 being the first to do so.
IE10 is no longer supported by Microsoft and previous versions can also be ignored. IE11 supports full ES5 but has zero support for any ES2015 features For IE11 to function with post ES5 code, transpiling is required.